motor limits updated outside of bluesky

[prjemian]

USAXS recently reported LimitError: Value 140.0 outside of range: [-113.4, 118.0]. This error comes up when the motor limits are changed outside of ophyd. The devices.EpicsMotorLimitsMixin class needs to capture and respond to CA monitor events on the limits.

Also noted in bluesky/ophyd#802

[prjemian]

The limits are contained in the ophyd.signal.EpicsSignalBase._metadata dictionary. Simply updating this dictionary is not sufficient, the initial values will be reset on the next motor move. But, there is a _metadata_changed() method that will make all the necessary changes. It needs the PV name and a dictionary of the new metadata.